Add Node 12 support

[markyleong]

The current version (1.8.0) has a dependency on node-sass#4.10.0, which is incompatible with Node 12: sass/node-sass#2632

Example error:
error: no matching function for call to ‘v8::Function::New(v8::Isolate*&, void (&)(const v8::FunctionCallbackInfo<v8::Value>&), #v8::Local<v8::Object>&)’